Hybrid sensor method

ABSTRACT

A method of determining a presence, concentration or change in concentration of a first or second material in an environment is disclosed. The method comprises measuring a response of a first sensor to the first and second material, wherein the first sensor is one of a metal oxide sensor, an electrochemical sensor, a photoionisation sensor, an infrared sensor, a pellistor sensor, an optical particle monitor, a quartz crystal microbalance sensor, a surface acoustic wave sensor, a cavity ring-down spectroscopy sensor, or a biosensor. The method further comprises measuring a response of a second sensor to the first and second material, wherein the second sensor is another one of a metal oxide sensor, an electrochemical sensor, a photoionisation sensor, an infrared sensor, a pellistor sensor, an optical particle monitor, a quartz crystal microbalance sensor, a surface acoustic wave sensor, a cavity ring-down spectroscopy sensor, a biosensor or a field effect transistor sensor. The method further comprises determining from first and second sensor measurements, a presence, concentration or change in concentration of the first or second material.

BACKGROUND

Embodiments of the present disclosure relate to methods and systems for sensing a target gas in an environment. More particularly, but not by way of limitation, some embodiments of the present disclosure relate to apparatus and methods for sensing a 1-methylcyclopropene (1-MCP) and/or ethylene.

Thin film transistors (TFTs) have been previously used as gas sensors. For example, such use of thin film transistors as gas sensors is described in Feng et al., “Unencapsulated Air-stable Organic Field Effect Transistor by All Solution Processes for Low Power Vapor Sensing” Scientific Reports 6:20671 DOI: 10.1038/srep20671 and Besar et al., “Printable ammonia sensor based on organic field effect transistor”, Organic Electronics, Volume 15, Issue 11, November 2014, Pages 3221-3230. In thin film transistor gas sensors, a semiconducting layer is in electrical contact with source and drain electrodes and a gate dielectric is disposed between the semiconducting layer and a gate electrode. Interaction of a target material with the TFT gas sensor may alter the drain current of the TFT gas sensor.

Metal oxide gas sensors have also been used to detect the presence of a target material in a gaseous environment. For example, metal oxide gas sensors have been described in Wang et al., “Metal Oxide Gas Sensors: Sensitivity and Influencing Factors”, Sensors 2010, 10(3) 2088-2106 DOI: 10.3390/s100302088. Metal oxide gas sensors detect concentration of various types of gases by measuring the resistance change of the metal oxide due to adsorption of gases.

Ethylene produced by plants can accelerate ripening of climacteric fruit, the opening of flowers, and the shedding of plant leaves. 1-methylcyclopropene (1-MCP) is known for use in inhibiting such processes.

It may be desirable to determine the presence and/or concentration of certain materials in an environment. However, a sensor used for this purpose may respond to one or more materials in the environment other than the target material; the concentration of background materials in the environment that the sensor responds to may change over time; or the response of the sensor to a target or background material may change as the sensor ages.

SUMMARY

In some embodiments there is provided a method of determining a presence, concentration or change in concentration of a first or second material in an environment. The method comprises measuring a response of a first sensor to the first and second material. The first sensor is one of a metal oxide sensor, an electrochemical sensor, a photoionisation sensor, an infrared sensor, a pellistor sensor, an optical particle monitor, a quartz crystal microbalance sensor, a surface acoustic wave sensor, a cavity ring-down spectroscopy sensor, or a biosensor. The method further comprises measuring a response of a second sensor to the first and second material. The second sensor is another one of a metal oxide sensor, an electrochemical sensor, a photoionisation sensor, an infrared sensor, a pellistor sensor, an optical particle monitor, a quartz crystal microbalance sensor, a surface acoustic wave sensor, a cavity ring-down spectroscopy sensor, a biosensor or a field effect transistor sensor. The method further comprises determining from first and second sensor measurements, a presence, concentration or change in concentration of the first or second material.

In some embodiments, the first or second material has been removed from the environment.

In some embodiments, the method further comprises applying a correction to the first sensor measurement of the first or second material based on the second sensor measurement of the same material.

In some embodiments, the method further comprises applying a correction to the second sensor measurement of the first or second material based on the first sensor measurement of the same material.

In some embodiments, the environment is a gaseous environment and gas drawn from the environment is desiccated and the response of the first and/or second sensors is a response to the desiccated gas.

In some embodiments, the desiccated gas is hydrated after desiccation and the response of the first and/or second sensors is a response to the hydrated gas.

In some embodiments, in response to determining that the first material is below a threshold, the first material concentration in the environment is increased.

In some embodiments, the first material is 1-methylcyclopropene.

In some embodiments, the second material is ethylene.

In some embodiments, there is provided a system comprising processor, a first sensor configured to respond to a first and a second material in an environment. The first sensor is one of a metal oxide sensor, an electrochemical sensor, a photoionisation sensor, an infrared sensor, a pellistor sensor, an optical particle monitor, a quartz crystal microbalance sensor, a surface acoustic wave sensor, a cavity ring-down spectroscopy sensor, or a biosensor. The system further comprises a second sensor configured to respond to a first and a second material in an environment. The second sensor is another of a metal oxide sensor, an electrochemical sensor, a photoionisation sensor, an infrared sensor, a pellistor sensor, an optical particle monitor, a quartz crystal microbalance sensor, a surface acoustic wave sensor, a cavity ring-down spectroscopy sensor, a biosensor or a field effect transistor sensor. The processor is configured to measure a response of the first sensor to the first and second material and measure a response of the second sensor to the first and second material. The processor is further configured to determine from first and second sensor measurements, a presence, concentration or change in concentration of the first and/or second material.

In some embodiments the system further comprises a source of the first material. In response to the first and/or second material being above or below a threshold the processor is further configured to release the first material into the environment.

DESCRIPTION OF THE DRAWINGS

The disclosed technology and accompanying figures describe some implementations of the disclosed technology.

FIG. 1 illustrates a system for determining the presence, concentration or change in concentration of a target material in an environment;

FIG. 2 is a process flow diagram according to some embodiments for determining the presence, concentration or change in concentration of a first and second target material in an environment using measurements of first and second sensors;

FIG. 3 is a graph of the response of a metal oxide sensor to ethylene in humid and dry conditions against time.

FIG. 4 is a graph of the percentage response of a metal oxide sensor against ethylene concentration in dry and humid conditions.

FIG. 5 is a graph of the response of six metal oxide sensors to 1 ppm 1-methylcyclopropene in an N₂ background against time;

FIG. 6 is a graph of the normalised response of a metal oxide sensor to 1 ppm 1-methylcyclopropene in air, or N₂ backgrounds in dry or humid conditions against time.

FIG. 7 is a graph of the normalised response of a metal oxide sensor to 1 ppm 1-methylcyclopropene in air, oxygen or N₂ backgrounds in dry or humid conditions against time.

The drawings are not drawn to scale and have various viewpoints and perspectives. The drawings are some implementations and examples. Additionally, some components and/or operations may be separated into different blocks or combined into a single block for the purposes of discussion of some of the embodiments of the disclosed technology. Moreover, while the technology is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the technology to the particular implementations described. On the contrary, the technology is intended to cover all modifications, equivalents, and alternatives falling within the scope of the technology as defined by the appended claims.

DETAILED DESCRIPTION

Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to.” As used herein, the terms “connected,” “coupled,” or any variant thereof means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, electromagnetic, or a combination thereof. Additionally, the words “herein,” “above,” “below,” and words of similar import, when used in this application, refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word “or,” in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list.

As used herein, by a material “over” a layer is meant that the material is in direct contact with the layer or is spaced apart therefrom by one or more intervening layers.

As used herein, by a material “on” a layer is meant that the material is in direct contact with that layer.

A layer “between” two other layers as described herein may be in direct contact with each of the two layers it is between or may be spaced apart from one or both of the two other layers by one or more intervening layers.

The teachings of the technology provided herein can be applied to other systems, not necessarily the system described below. The elements and acts of the various examples described below can be combined to provide further implementations of the technology. Some alternative implementations of the technology may include not only additional elements to those implementations noted below, but also may include fewer elements.

These and other changes can be made to the technology in light of the following detailed description. While the description describes certain examples of the technology, and describes the best mode contemplated, no matter how detailed the description appears, the technology can be practiced in many ways. Details of the system may vary considerably in its specific implementation, while still being encompassed by the technology disclosed herein. As noted above, particular terminology used when describing certain features or aspects of the technology should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the technology with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the technology to the specific examples disclosed in the specification, unless the Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the technology encompasses not only the disclosed examples, but also all equivalent ways of practicing or implementing the technology under the claims.

To reduce the number of claims, certain aspects of the technology are presented below in certain claim forms, but the applicant contemplates the various aspects of the technology in any number of claim forms. For example, while some aspect of the technology may be recited as a computer-readable medium claim, other aspects may likewise be embodied as a computer-readable medium claim, or in other forms, such as being embodied in a means-plus-function claim.

In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of implementations of the disclosed technology. It will be apparent, however, to one skilled in the art that embodiments of the disclosed technology may be practiced without some of these specific details.

The techniques introduced here can be embodied as special-purpose hardware (e.g., circuitry), as programmable circuitry appropriately programmed with software and/or firmware, or as a combination of special-purpose and programmable circuitry. Hence, embodiments may include a machine-readable medium having stored thereon instructions which may be used to program a computer (or other electronic devices) to perform a process. The machine-readable medium may include, but is not limited to, floppy diskettes, optical disks, compact disc read-only memories (CD-ROMs), magneto-optical disks, ROMs, random access memories (RAMs), erasable programmable read-only memories (EPROMs), electrically erasable programmable read-only memories (EEPROMs), magnetic or optical cards, flash memory, or other type of media/machine-readable medium suitable for storing electronic instructions. The machine-readable medium includes non-transitory medium, where non-transitory excludes propagation signals. For example, a processor can be connected to a non-transitory computer-readable medium that stores instructions for executing instructions by the processor.

Sensors such as gas, liquid or particulate sensors may suffer from drift, i.e. the signal produced may change over time without any change in the environment (e.g. composition of the environment or changes in pressure or temperature). This may limit the lifetime of the sensor and/or accuracy of its measurements. The present inventors have found that changes arising for such drift may be compensated for using sensor systems, apparatus and methods as described herein.

System Overview

Referring to FIG. 1 , a sensor system according to some embodiments for sensing the presence, concentration or change in concentration of a target material in an environment, for example a gaseous or liquid environment, is shown. The system 1 includes a sensor apparatus 2 operatively connected to a controller 3. The sensor apparatus 2 includes a first sensor 6 and a second sensor 7. The first and second sensors 6, 7 are configured to respond to a target material in the environment to allow them to detect the presence, concentration or change in concentration of a target material in an environment. The first and second sensors 6, 7 may be configured to respond to a first and a second target material.

The target material may be a volatile organic compound. The target material may be an evaporated organic compound which may have a boiling point at the pressure of the environment, which is higher than the temperature of the environment, e.g. an organic compound having a boiling point above 25° C. at one atmosphere.

The target material may be a gas, a liquid, or a particulate in suspension in a fluid. In some embodiments, the target material is an alkene. The alkene may be an acyclic alkene, e.g. ethylene. The alkene may comprise an alkene group substituted with an aromatic group, e.g. styrene. The alkene may be cyclic, e.g. 1-MCP.

In some embodiments, the target material is an alkane, e.g. methane.

In some embodiments, the target material is an ester. The ester may be a C1-10 alkyl ester or C1-10 alkanoate ester, optionally C1-10 alkyl-C1-10 alkanoate esters, e.g. ethyl acetate; ethyl butanoate; ethyl hexanoate; propyl acetate; butyl acetate; butyl butanoate; butyl hexanoate; pentyl acetate; hexyl acetate; hexyl butanoate; hexyl hexanoate; 2-methylpropyl acetate; 2-methylbutyl acetate; ethyl 2-methylbutanoate; butyl 2-methylbutanoate; and hexyl 2-methylbutanoate.

In some embodiments, the target material is a polar compound. The target polar compound may be a hydrocarbon which does not have a mirror plane bisecting a carbon-carbon bond of the hydrocarbon. The target polar compound may have dipole moment of greater than 0.2 Debyes optionally greater than 0.3 or 0.4 Debyes. The target polar compound may be 1-MCP. The target material may be, for example, 1-methylcyclopropene (1-MCP) and/or ethylene.

Optionally the sensor apparatus 2 may include first and second filter chambers 10, 11, which may include, for example a filter. The filter may be a filter material, a filter mesh, a chemical filter, a scrubber or a filter device and the like. A suitable filter will be selected according to the material needed to be removed or reduced in concentration. The filter chambers 10, 11 may be arranged between an optional fluid inlet 12 and the first and second sensors 6, 7 respectively. In use, this would allow the filters 10, 11 to filter the environment before the fluid from the environment reached the first and second sensors 6, 7.

If the environment to be sensed is a gaseous environment, the filter may either include, comprise or consist of a desiccant or dehumidifier. The desiccant or dehumidifier may be any suitable desiccant, for example, a molecular sieve which may have a size selected to remove water but not a target material, e.g. a molecular sieve having a pore size of less than 4 Å, e.g. 3 Å if the target gas is an organic compound such as an alkene, e.g. 1-MCP or ethylene; an alcohol, e.g. ethanol; or CO₂. The desiccant may be a solid state salt (e.g. a solid metal salt). The salt may be an ammonium, alkali, alkali earth or transition metal salt. The salt may be, without limitation, a halide, hydroxide, sulfate, acetate, dichromate, formate or nitrate. Exemplary salts include, without limitation, NH₄NO₃, (NH₄)₂SO₄, LiCl, NaCl, MgCl₂, KCl, KOH, KBr, KI, NaBr, Mg(NO₃)₂, NaNO₃, KNO₃, sodium or potassium acetate, sodium or potassium dichromate, calcium formate and copper sulfate. The desiccant may be silica gel which removes water from the sample and may also remove target material from the sample.

If the environment to be sensed is a gaseous environment, the sensor apparatus 2 may further include first and second humidifiers 14, 15. The humidifiers 14, 15 may be arranged between the first and second filter chambers 10, 11 and the first and second sensors 6, 7 respectively. In use, this would allow the humidifiers 14, 15 to rehumidify or hydrate and/or control the humidity of a gas before the reaches the sensors 6, 7. The first and second humidifiers 14, 15 may be, for example, a water reservoir or a saturated salt solution. The first and second humidifiers 14, 15 may comprise a gel humidifier. The gel may or may not contain a salt. A preferred humidity of a gas reaching the first or second sensors 6, 7 is in the range of 60-95% in the case where the sensor is an organic thin film transistor.

The controller 3 may further include a processor 17, system memory 18, and an interface 19, (e.g. a display) to allow the controller 3 to be programmed, for example, by a human. The controller 3 may include additional peripheral devices for recording environmental conditions, for example, a thermometer (not shown) and a hygrometer (not shown) and the like.

The sensor apparatus 2 may be in wired or wireless communication with the controller 3 configured to receive measurements from the first and/or second sensors 6, 7. The processor 17 may be configured to determine the presence, concentration and/or change in concentration of the target material from the received sensor measurements. The system memory 18 may store the sensor measurements, and/or other environmental information recorded by the system 1.

In the case where the target material is 1-MCP, the sensor apparatus may be configured to send a signal to the interface 19 or controller 3 if 1-MCP concentration as determined from measurements of the sensor apparatus falls below a threshold concentration. The controller 3 may be configured to activate a 1-MCP source for release of 1-MCP into the environment upon receiving a signal from the processor 17 that 1-MCP concentration has fallen below a threshold concentration.

The system 1 may further include a fluid outlet 20 for expelling fluid from the sensor apparatus 2. Optionally, the sensor system 1 is configured for fluid connection to a material 21 which may be a non-target material source, that is, a material which it is not intended to be sensed in the environment. The non-target material source may be a source of a fluid to which the first and/or second sensors do not respond, for example nitrogen, and in which a sensor is able to desorb a target material.

Method of Sensing

The first sensor 6 may respond differently in the presence of a target material than the second sensor 7. For example, a first sensor 6 may be a metal oxide-based sensor and the second sensor may be field effect transistor-based sensor (e.g. a thin film transistor (TFT) sensor). In an environment which may contain ethylene and/or 1-MCP, a metal oxide-based sensor responds more quickly and with greater magnitude to 1-MCP than a field effect-based transistor, but may be subject to greater cross-sensitivity from volatiles, including ethylene. A metal oxide-based sensor in such an environment may be subject to drift over time, meaning the presence, concentration or change in concentration of 1-MCP may not be accurate after continuous sensing for a few hours.

Field effect transistor-based sensors are less sensitive to ethylene in the background of an environment than metal oxide-based sensor due to their different operation mechanism. Field effect transistor-based sensors may be insensitive to ethylene in the background of an environment. For example, contact resistance limited organic thin film transistor sensors (OTFTs) amplify the work function shift induced by 1-MCP molecules on the Au-based source and drain contacts, which does not change upon exposure to ethylene. As such, a field effect transistor-based sensor enables selective sensing of 1-MCP and ethylene (i.e. discrimination between these two materials). However, while field effect transistor-based sensors such as an OTFT sensor may be accurate and not as effected by volatiles in the background of the environment they are deployed in, they may not be reliable at sensing for the long periods which may be required for certain applications, such as sensing 1-MCP in a storage unite containing fruit.

The inventors made the surprising discovery that by using two different types of sensors such as these (e.g. a field effect transistor-based sensor and a metal oxide-based sensor), it is possible to accurately and reliably determine the presence, concentration or change in concentration of a material in an environment. For example, the accurate and reliable sensing of both 1-MCP and ethylene in a food or flower store may be achieved. Accurate and reliable sensing may not be possible if using either of these sensors on their own.

Referring to FIG. 2 , a response of a first sensor 6 to a first and/or second material in an environment is measured (step S1). The first sensor 6 is one of a metal oxide sensor, an electrochemical sensor, a photoionisation sensor, an infrared sensor, a pellistor sensor, an optical particle monitor, a quartz crystal microbalance sensor, a surface acoustic wave sensor (SAWS), a cavity ring-down spectroscopy sensor (CRDS) or a biosensor. A response of a second sensor 7 to a first and/or second material in an environment is measured (step S2). The second sensor 7 is another of a metal oxide sensor, an electrochemical sensor, a photoionisation sensor, an infrared sensor, a pellistor sensor, an optical particle monitor, a quartz crystal microbalance sensor, a surface acoustic wave sensor, a cavity ring-down spectroscopy sensor, a biosensor or a field effect transistor sensor.

The second sensor 7 may be of a different type of sensor to the first sensor 6. The second sensor may be the same type of sensor as the first sensor, but the first and second sensors have different sensitivities, speeds of response and/or specificities to the target material. For example both the first and second sensors may be electrochemical sensors, but the first sensor may respond with greater magnitude and more quickly to a target material than the second sensor, and may also respond more readily to non-target materials in the background of the environment.

Using the measurements from the first and second sensors 6, 7 a presence, concentration or change in concentration of the first or second material is determined (step S3). If environment sensing is still required, the method may return to step S1. The method may be implemented by the controller 3 and/or processor 17 in the system 1.

In some embodiments, the first and second sensors 6, 7 may measure the response to both the first and second material, and the measurement of either the first or second material by one sensor may be used to correct the measurement of the same material by the other sensor. In some embodiments, the first or second material is only present in the environment for some of the time during which measurement takes place. For example, in a fruit or flower storage container, ethylene may not be present when the fruit is first stored in the container, but the concentration increases over time.

Examples of sensors which may be more sensitive to the first or second material (for example, having a greater magnitude of response to changes in presence or concentration) and/or have a faster response to the first or second material are metal oxide sensors, photoionisation sensors, electrochemical sensors or pellistor sensors. These types of sensors may also be sensitive to other materials in the environment which are not the first or second material. For example, in a gaseous environment, these sensors may be sensitive to volatiles and/or ethylene present. The sensitivity to these other materials in the environment may also be referred to as cross-sensitivity. Cross-sensitivity may result in the measurement of a sensor in response to the first or second material being inaccurate or unreliable.

Examples of sensors which may be less sensitive to the first or second material (for example having a smaller magnitude of response to changes in presence or concentration) and/or have a slower response to the first or second material are electrochemical sensors, infrared sensors, organic thin film transistors or other field effect transistor-based sensors, biosensors, quartz crystal microbalance sensors, surface acoustic wave sensors, cavity ring-down spectroscopy sensors or optical particle monitors. QCMB and SAWS sensors may have a functionalised coating applied to them in order to reduce cross-sensitivity. While these types of sensor may have a lower magnitude of response to changes in presence or concentration, they may be less susceptible to effects from other material in the environment which are not the first or second material they are sensing in. These types of sensors can be referred to as being a more selective type of sensor.

The first and second sensors 6, 7 may be calibrated to each other, so that the measurements taken form the first sensor 6 in response to the first material may be used to correct or adjust the measurements taken from the second sensor 7 of the first material. This may be done by measuring the response of the first and second sensors to a range of concentrations of the first material to calculate the correction (e.g. a subtraction or addition in the concentration of the first material). The first and second sensors may also be calibrated to each other for their response to the second material. Calibration of the first and second sensors 6, 7 to each of the first and second materials may be in the presence or absence of the other material.

Semiconductor sensors may comprise an organic semiconductor, an inorganic semiconductor or a combination thereof. The organic semiconductor may be polymeric or non-polymeric.

Optionally, the first or second materials may be removed from the environment prior to coming into fluid communication with either the first or second sensors 6, 7. The first and second filter chambers 10, 11 may contain filter material to optionally remove the first or second target material.

The first sensor 6 may be calibrated or a correction calculated and applied by using measurements from the second sensor 7, and/or measurements from the first sensor 6 where a target material has been removed. The second sensor 7 may be calibrated or a correction calculated and applied by using measurements from the first sensor 6, and/or measurements from the second sensor 7 where a target material has been removed.

One of the applications for this hybrid sensor method would be for use as an ethylene sensor to monitor the concentration inside a fruit store or shipping container. This may help for monitoring the ripening process, and enable interventions (e.g. removal of fruit or application of 1-MCP treatment). For example, apples might be stored for up to one year inside a controlled atmosphere store, and should not be opened during this time. The increase in ethylene concentration during storage is likely to be very slow and in addition there will be significant changes in background conditions, such as fruit volatile levels. These can be challenging conditions for many gas sensors, as they can suffer from drift and cross-sensitivity issues. Nevertheless, by occasionally removing a target material from the environment, a sensor may be able to desorb material and provide a more accurate reading. A suitable filter for ethylene may be a molecular sieve, for example, a 4 Å, which may not remove larger fruit volatile molecules.

A 4 Å molecular sieve also removes 1-MCP from a gaseous environment. 1-MCP is also relevant for fruit storage applications, as this may be used as a treatment for stopping or slowing the ripening process by capping ethylene receptors. 1-MCP may be sensed (e.g. at 0.1-10 ppm) by using organic thin film transistors (OTFTs) with Au contacts, unless they have had a thiol treatment, that is, thiol disposed onto the contact surface. There are many other types of sensors that can also be used for sensing 1-MCP (in addition to gas chromatography (GC)), for example, metal oxide sensors, infrared, electrochemical (EC) and photoionization detector (PID).

Metal oxide sensors may be more sensitive and/or accurate in a dry environment. As discussed previously, if the environment is a gaseous environment, the sample to be sensed by the first and second sensors may be desiccated prior to the gas being in fluid communication with the first and second sensors. The environment may also be rehumidified or hydrated, or have its humidity controlled to be within a certain range prior to being in fluid communication with the first and second gas sensors.

If it is determined that one of the target materials is below a threshold, it may trigger the release of more of that target material into the environment. For example, in a fruit or flower storage unit, where 1-MCP is used to suppress the ripening of fruit or the opening of flowers, if the method determines that the amount of 1-MCP or the ratio of 1-MCP to ethylene falls below a threshold, it may trigger the release of 1-MCP into the environment.

Referring to FIG. 3 , the response of a metal oxide-based sensor (Figaro TGS823) to two hour exposures to different concentrations of ethylene (between 1 and 50 ppm) in nitrogen background with 1% oxygen in dry and humid conditions is shown. The counts on the y-axis represent the voltage read be a 10 bit (1024 count) analogue to digital convertor (ADC) by a microcontroller. The voltage read changes as the sensor element resistance changes.

Referring to FIG. 4 , the magnitude of response of a metal oxide-based sensor (Figaro TGS823) to different concentrations of ethylene (between 1 and 50 ppm) in nitrogen background with 1% oxygen in dry and humid conditions is shown. The concentration dependence is non-linear, but similar at low and high relative humidity background.

Referring to FIG. 5 , the 1-MCP response (in a humid N₂ background) is plotted for six commercially available metal oxide-based sensor components from Figaro and AMS assembled in a multi-sensor chamber.

Cross-sensitivity to ethylene may be a problem for many sensors with the exception of some OTFTs, for example, top gate OTFTs as described in WO 2020/021251 A1, the contents of which are incorporated herein by reference.

Table 1 shows the change in the response to 1-MCP with a constant ethylene concentration 100 ppm of ethylene in the background.

TABLE 1 Change in Sensor response (%) TGS823 MOx −98 AS-MLV −76 AQ-VOC PID −86 AQ-VOC EC −100 OTFT −2

The metal oxide-based sensors, the photoionization detector and electrochemical sensors all show much lower sensitivity (and therefore magnitude of response) in a background with ethylene present, which is likely to be the case in fruit stores and shipping containers.

As explained earlier, the reason OTFTs are insensitive to the ethylene background, is because the mechanism of operation is different. Contact resistance limited OTFTs are amplifying the work function shift induced by the 1-MCP molecules on the Au-based source and drain contacts, which does not change upon exposure to ethylene.

Referring to FIGS. 6 and 7 , the normalised response to two-hour exposures of 1 ppm 1-MCP in air and in N₂ and in dry and humid backgrounds is plotted for two commercially available metal oxide sensors CC811 and TGS823 respectively. The plots indicate that the sensors are more sensitive to 1-MCP in a dry environment with a background of N₂.

By combining a more sensitive and/or faster responding sensor, but one that has greater cross-sensitivity (e.g. a metal oxide-based sensor) with a less sensitive, sensor that has a greater selectivity or specificity (e.g. a field effect transistor-based sensor), selective measurements of a first material's (e.g. 1-MCP) presence or concentration and a second material's (e.g. ethylene) presence or concentration can be achieved in different ways, by using the measurements from each sensor to determine an accurate concentration of the first and second materials. 

1. A method of determining a presence, concentration or change in concentration of a first or second material in an environment, the method comprising: measuring a response of a first sensor to the first and second material, wherein the first sensor is one of a metal oxide sensor, an electrochemical sensor, a photoionisation sensor, an infrared sensor a pellistor sensor, an optical particle monitor, a quartz crystal microbalance sensor, a surface acoustic wave sensor, a cavity ring-down spectroscopy sensor or a biosensor; measuring a response of a second sensor to the first and second material, wherein the second sensor is a another one of a metal oxide sensor, an electrochemical sensor, a photoionisation sensor, an infrared sensor or a pellistor sensor, an optical particle monitor, a quartz crystal microbalance sensor, a surface acoustic wave sensor, a cavity ring-down spectroscopy sensor, a biosensor or a field effect transistor sensor; determining from first and second sensor measurements, a presence, concentration or change in concentration of the first or second material.
 2. The method of claim 1 wherein the first or second material has been removed from the environment.
 3. The method of claim 1 further comprising applying a correction to the first sensor measurement of the first or second material based on the second sensor measurement of the same material.
 4. The method of claim 1 further comprising applying a correction to the second sensor measurement of the first or second material based on the first sensor measurement of the same material.
 5. The method of claim 1 wherein the environment is a gaseous environment and gas drawn from the environment is desiccated and the response of the first and/or second sensors is a response to the desiccated gas.
 6. The method of claim 5 wherein the desiccated gas is hydrated after desiccation and the response of the first and/or second sensors is a response to the hydrated gas.
 7. The method of claim 1 wherein in response to determining that the first material is below a threshold, the first material concentration in the environment is increased.
 8. The method of claim 1 wherein the first material is 1-methylcyclopropene.
 9. The method of claim 1 wherein the second material is ethylene.
 10. A system comprising: a processor; a first sensor configured to respond to a first and a second material in an environment, wherein the first sensor is one of a metal oxide sensor, an electrochemical sensor, a photoionisation sensor, an infrared sensor, a pellistor sensor, an optical particle monitor, a quartz crystal microbalance sensor, a surface acoustic wave sensor, a cavity ring-down spectroscopy sensor or a biosensor; a second sensor configured to respond to a first and a second material in an environment, wherein the second sensor is another of a metal oxide sensor, an electrochemical sensor, a photoionisation sensor, an infrared sensor a pellistor sensor, an optical particle monitor, a quartz crystal microbalance sensor, a surface acoustic wave sensor, a cavity ring-down spectroscopy sensor, a biosensor or a field effect transistor sensor; wherein the processor is configured to: measure a response of the first sensor to the first or second material; measure a response of the second sensor to the first or second material; and determine from first and second sensor measurements, a presence, concentration or change in concentration of the first and/or second material.
 11. The system according to claim 10, further comprising a source of the first material; and wherein in response to the first or second material being above or below a threshold the processor is further configured to release the first material into the environment. 